74HC/HCT4046A

The 74HC/HCT4046A are high-speed Si-gate CMOS devices and are pin compatible with the "4046" of the "4000B" series. They are specified in compliance with JEDEC standard no. 7A.

The 74HC/HCT4046A are phase-locked-loop circuits that comprise a linear voltage-controlled oscillator (VCO) and three different phase comparators (PC1, PC2 and PC3) with a common signal input amplifier and a common comparatorinput.

The signal input of the 74HC/HCT4046A can be directly coupled to large voltage signals, or indirectly coupled (with a series capacitor) to small voltage signals. A self-bias input circuit keeps small voltage signals within the linear region of the input amplifiers. With a passive low-pass filter, the "4046A" forms a second-order loop PLL. The excellent VCO linearity is achieved by the use of linear op-amp techniques.

The VCO requires one external capacitor C1 (betweenCA and C1B)and one external resistor R1 (betweenR1and GND) or two external resistors R1 and R2(between R1and GND, and R2and GND). Resistor R1 and capacitor C1 determine the frequency range of the VCO. Resistor R2 enables the VCO to have a frequency offset if required.

The high input impedance of the VCO simplifies the design of low-pass filters by giving the designer a wide choice of resistor/capacitor ranges. In order not to load the low-pass filter, a demodulator output of the VCO input voltage isprovided at pin10 (DEM_OUT). In contrast to conventional techniques where the DEM _OUT voltage is one threshold voltage lower than the VCO input voltage, here the DEM _OUT voltage equals that of the VCO input. If DEM_OUT is used, a load resistor (RS)should be connected from DEM_OUT to GND; if unused, DEM_OUT  of the 74HC/HCT4046A should be left open. The VCO output (VCO_OUT)can be connected directly to the comparator input (COMP_IN), or connectedvia a frequency-divider. The VCO output signal has a duty factor of 50% (maximum expected deviation 1%), if the VCO input is held at a constant DC level. A LOW level at the inhibit input (INH)enables the VCO and demodulator, while a HIGH level turns both off to minimize standby power consumption.

The only difference between the HC and HCT versions of the 74HC/HCT4046A is the input level specification of the INH input. This input disables the VCO section. The sections of the comparator are identical, so that there is no difference in the SIG_IN(pin14) or COMP_IN (pin3) inputs between the HC and HCT versions.